1. Field of the Invention
The present invention relates to a flat microtip display screen.
2. Discussion of the Related Art
FIG. 1 partially shows, in cross-sectional view, the structure of a flat microtip display screen of the type to which the present invention relates.
Such a screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 having holes 4 at the locations of the microtips 2. Cathode 1 is placed facing a cathodoluminescent anode 5, a glass substrate 6 of which forms the screen surface.
The operating principle and the detail of the constitution of such a microtip screen are described in U.S. Pat. No. 4,940,916 of the Commissariat a l'Energie Atomique.
Cathode 1 is formed, on a glass substrate 7, of cathode conductors 8 organized in columns. These conductors 8 are generally coated with a resistive layer (not shown) for homogenizing the electronic emission. Cathode 1 is associated with grid 3, with a layer 9 being interposed to insulate the cathode conductors 8 from grid 3. The holes 4 are bored into grid layer 3 and insulating layer 9 for receiving the microtips 2 which are formed on the resistive layer. Grid 3 is organized in rows, the intersection of a row and of a column of the cathode defining a pixel.
This device uses the electric field created between cathode 1 and grid 3 for the extraction of electrons from the microtips 2 towards phosphor elements 10 of anode 5, the electrons crossing an empty space 11. Phosphor elements 10 are deposited on electrodes 12, formed of a transparent conductive layer such as indium and tin oxide (ITO).
In the case of a color screen, anode 5 is provided with alternate bands of phosphor elements 10, each corresponding to a color (Red, Green, Blue). Each band is electrically insulated from the two neighboring bands. The bands are arranged to be parallel to the cathode columns 8, a group of three bands (one per color) facing a cathode column. The sets of red, green, blue bands are selectively biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode/grid are selectively directed towards the phosphor elements 10 facing each of the colors.
In the case of a monochrome screen, the anode is generally comprised of a plane of phosphor elements or of two sets of alternate bands of the same color.
FIG. 2 partially shows an electronic emission microtip cathode associated with a grid for extracting the emitted electrons, to illustrate the addressing of the cathode and of the grid during the operation of a microtip screen. The anode (5, FIG. 1) and the insulating layer (9, FIG. 1) between cathode 1 and grid 3 have not been shown, for clarity. Similarly, only a few microtips 2 have been shown at the intersection of a row L of grid 3 and of a column K of cathode 1. In practice, there are several thousands of microtips per screen pixel.
The display of an image is performed during a frame period (for example 20 ms) by properly biasing the anode, cathode 1 and grid 3 by means of an electronic control circuit (not shown). For a color screen, the sets of bands of phosphor elements (10, FIG. 1) are sequentially brought to a potential enabling to attract the electrons. This potential depends on the distance (empty space 11) which separates the cathode/grid from the anode and is, for example, higher than 300 volts. The bands 10 are biased during a frame, for example of bands of same color, that is, for a sub-frame duration corresponding to one third of the frame period (for example, 6.6 ms). The display is performed line by line, by sequentially biasing (for example, to 80 volts) the rows L for a duration (for example 30 .mu.s) corresponding to the duration of a sub-frame divided by the number of rows of grid 3. While a row L is biased, the columns K are brought to respective potentials between a maximum emission potential and a no-emission potential (for example, respectively 0 and 30 volts), to set the brightness of the pixels defined by the intersection of these columns and of the considered row. The biasing of the columns K changes for each new row L of the line scanning. The choice of the values of the biasing potentials is related to the characteristics of the phosphor elements 10 and of the microtips 2. Conventionally, below a voltage differential of 50 volts between cathode 1 and grid 3, there is no electronic emission, and the maximum emission corresponds to a voltage differential of 80 volts.
The microtips see their emissivity decrease as they are being used. The screens thus have a brightness which decreases with time. This decrease is significant enough not to be negligible and influences the lifetime of the screen.